Iraq is over. Iraq has not yet begun. Two conclusions from the American debate about Iraq, which dominates the media in the US to the exclusion of almost any other foreign story. Iraq is over insofar as the American public has decided that most US troops should leave. In a Gallup poll earlier this month, 71% favoured "removing all US troops from Iraq by April 1 of next year, except for a limited number that would be involved in counter-terrorism efforts". CNN's veteran political analyst Bill Schneider observes that in the latter years of the Vietnam war, the American public's basic attitude could be summarised as "either win or get out". He argues that it's the same with Iraq. Despite George Bush's increasingly desperate pleas, most Americans have now concluded that the US is not winning. So get out.

Since this is a democracy, their elected representatives are following where the people lead. Whatever the result of the latest round of Congressional position play - which included an all-night marathon on the floor of the Senate from Tuesday to Wednesday this week as Democrats attempted to outface a Republican filibuster - no one in Washington doubts that this is the way the wind blows. Publicly, there's still a sharp split along party lines, but leading Republicans are already breaking ranks to float their own phased troop reduction plans, together with proposals for partitioning Iraq between Sunni, Shia and Kurds.

Bush says he's determined to give the commanding general in Iraq, David Petraeus, exactly the troop levels he asks for when he reports back this September, and the White House may hold the line for now against a Democrat-controlled Congress. Leading Republican contenders for the presidency are still talking tough. However, the most outspoken protagonist of hanging in there to win in Iraq, John McCain, has seen his campaign nosedive.

Even if the next president is a hardline Republican, all the current Washington betting will be confounded if he does not, at the very least, rapidly reduce the number of US troops in Iraq. After all, that's what the American people plainly say they want - and so, incidentally, did 72% of American troops serving in Iraq, according to a Zogby survey conducted early last year. In fact, the boys themselves said they wanted to come home in the course of 2006.

The American people's verdict is remarkably sharp on other aspects of the Iraq debacle. Asked who they blamed most for the present situation in Iraq, 40% of those polled for Newsweek said the White House, and another 13% said Congress. In a poll for CNN, 54% said the US's action in Iraq was not morally justified. In one conducted for CBS, 51% endorsed the assessment - shared by most of the experts - that American involvement in Iraq is creating more terrorists hostile to the US rather than reducing their number. If once Americans were blind, they now can see. For all its plenitude of faith, this is a reality-based nation.

So Iraq is over. But Iraq has not yet begun. Not yet begun in terms of the consequences for Iraq itself, the Middle East, the US's own foreign policy and its reputation in the world. The most probable consequence of rapid US withdrawal from Iraq in its present condition is a further bloodbath, with even larger refugee flows and the effective dismemberment of the country. Already some 2 million Iraqis have fled across the borders and more than 2 million are internally displaced. Now a pained and painstaking study from the Brookings Institution argues that what its authors call "soft partition", involving the peaceful, voluntary transfer of an estimated 2 to 5 million Iraqis into distinct Kurdish, Sunni and Shia regions, under close US military supervision, would be the lesser evil. The lesser evil, that is, assuming that all goes according to plan and that the American public is prepared to allow the troops to stay in sufficient numbers to accomplish that thankless job - two implausible assumptions. A greater evil is more likely.

In an article for the web magazine Open Democracy, the Middle East specialist Fred Halliday spells out some regional consequences. Beside the effective destruction of the Iraqi state, these include the revitalising of militant Islamism and enhancement of the international appeal of the al-Qaida brand; the eruption for the first time in modern history of internecine war between Sunni and Shia - "a trend that reverberates in other states of mixed confessional composition"; the alienation of most sectors of Turkish politics from the west, and the stimulation of authoritarian nationalism there; the strengthening of a nuclear-hungry Iran; and a new regional rivalry, pitting the Islamic Republic of Iran and its allies, including Syria, Hizbullah and Hamas, against Saudi Arabia, Egypt and Jordan.

For the US itself, the world is now, as a result of the Iraq war, a more dangerous and hostile place. At the end of 2002, what is sometimes tagged al-Qaida Central in Afghanistan had been virtually destroyed and there was no al-Qaida in Iraq. In 2007, there is an al-Qaida in Iraq; parts of the old al-Qaida are creeping back into Afghanistan; and there are al-Qaida emulator groupuscules spawning elsewhere, notably in Europe. Osama bin Laden's plan was to get the US to overreact and over-reach itself. With the invasion of Iraq, President Bush fell slap-bang into that trap. The US government's own latest national intelligence estimate, released earlier this week, suggests that al-Qaida in Iraq is now among the most significant threats to the security of the American homeland.

Americans have probably not yet fully woken up to the appalling fact that, after a long period in which the first motto of their military was "no more Vietnams", they face another Vietnam. There are many important differences, of course, but the basic result is similar. The mightiest military in the world fails to achieve its strategic goals and is, in the end, politically defeated by an economically and technologically inferior adversary.

Even if there are no scenes of helicopters evacuating Americans from a flat roof of the US embassy in Baghdad, there will surely be totemic photographic images of national humiliation as the US struggles to extract its troops and all the heavy equipment it has poured into the country, perhaps this time an image snapped on a mobile phone and posted on the internet. Abu Ghraib and Guantánamo have done terrible damage to America's reputation for being humane; this defeat will convince more people around the world that it is not even all that powerful. And Bin Laden, still alive, will claim another victory over the death-fearing weaklings of the west.

In history, the most important consequences are often the unintended ones. We do not yet know the longer term unintended consequences of Iraq. Maybe there is a silver lining hidden somewhere in this cloud. But so far as the human eye can see, the likely consequences of Iraq range from the bad to the catastrophic. Looking back over a quarter-century of writing about international affairs, I can not recall a more comprehensive and avoidable man-made disaster. · Timothy Garton Ash's column returns in Septemberwww.timothygartonash.com